Unlike conventional hearing testing techniques which require the cooperation and participation of the subject or patient, Auditory Brainstem Response (ABR) testing senses the patient's hearing response automatically through the direct measurement of bioelectrical activity at the subject's brainstem. The use of Auditory Brainstem Response recordings has now become routine in the evaluation of hearing. They are particularly useful where difficulty would be encountered in conventional testing or where additional information is required beyond that available from conventional testing. Examples include infants who are uncooperative or are too young to respond consistently, foreign-language speaking adults, and suspected 8th nerve tumor patients. The ABR instrument provides an objective measure of the operation of the auditory system by using computer averaging to detect the small electrical potentials (typically less than a microvolt) generated on the scalp and near the ear when a click or tone pip at the ear causes a sequence of more or less synchronized volleys of neural firings along the auditory pathway. Computer averaging has long been used for stimulus/response measurement in patients. The technique involves a) the repetitive stimulus of the patient through one or more of his nerve systems (i.e., eye, ear, touch, etc.), b) the detection of the body's response through remotely located electrodes contacting or penetrating the skin and c) the repetitive sampling or "averaging" of the detected signal in synchronism with the stimulus so as to remove the background noise that is typically many dB greater than each detected signal.
Several years ago, the applicant and others saw three unsolved problems in ABR instrumentation:
1. The electromagnetic output of traditional supra aural input headphones such as the Telephonics TDH-39 introduced an artifact into the output recording that was often impossible to separate from a real response. PA1 2. Patient preparation included vigorous scrubbing (often resulting in lacerations) of the skin. The ABR pickup typically employed EEG electrode cups filled with silver-chloride paste, taped down over the scrubbed skin area. Several minutes were often required to prepare and apply the electrodes in order to keep the contact resistance below 5000 ohms, as was typically required in these tests. PA1 3. Even with low-impedance electrode preparations, electrical interference from radio stations, fluorescent lights, diathermy machines and the like sometimes made it completely impossible to obtain useful ABR recordings.
The applicant has previously described insert earphones which successfully solved the first problem, specifically in U.S. Pat. No. 4,677,679 dated July 5, 1984 and U.S. Pat. No. 4,766,753 dated Oct. 4, 1985, and a low-cost earcanal electrode that has simplified the electrode preparation in many cases due to the increased signal levels resulting from electrical pickup closer to the cochlea as described in U.S. Pat. No. 4,781,196 dated Nov. 1, 1988. These patents are incorporated herein by reference and form a part of the present disclosure.
A solution to the interference problem was initially approached by attempting to locate a high-input-impedance amplifier close to the electrodes, in the belief that a lower output impedance driving the cables connecting the electrodes to the ABR equipment would alleviate the problem. After continuing failures with this approach, we built an electrically-quiet "BATMAN" (Brainstem Amplifier Test MANikin) with salty jello for brains. Tests with this manikin convinced us that our high-input-impedance electrode amplifier only indirectly tackled the real nemesis of clean bioelectric recordings: The human body acts as an efficient antenna for pickup of extraneous electromagnetic interference signals (EMI), some of which can amount to tens of volts in magnitude.
Measures taken in good ABR equipment include the use of differential input amplifiers to obtain extraordinarily high common-mode-rejection ratios and large common-mode-input voltage ranges at high frequencies. These have been brute force (and in some environments regularly unsuccessful) attempts to avoid the contamination of the averaged signal from EMI pickup. The use of light-coupled isolation amplifiers to solve this and similar problems is well known, and light-coupled amplifier-transmitter-receiver-demodulator systems are commonly available. But their power consumption and cost has hitherto prevented their common use in ABR and similar equipment.